A generator may be used to convert mechanical power developed by a motor or an engine into electrical power. Such a generator has a housing, a stator provided within the housing, and a rotor magnetically coupled to the stator. When used in an aircraft, a generator converts mechanical power developed by an engine into electrical power used in the various systems of the aircraft.
The housing of a generator is typically composed of a relatively lightweight material, such as aluminum or magnesium alloy, while the stator typically comprises a plurality of relatively thin iron sheets laminated together with an adhesive, such as epoxy. The stator is provided with an initial axial load pressure, or axial preload, to force the sheets to remain in contact with each other. The stator is typically mounted within the housing by bolts which pass through holes in the stator housing and into the stator. One or more sets of bolts may be provided radially about the circumference of the stator housing, such as 120.degree. apart.
When used in an aircraft, a generator has a relatively wide temperature range through which it is exposed, for example from -65.degree. F. to +450.degree. F. Because the ferrous material of the stator typically has a coefficient of radial thermal expansion that is substantially different than that of the stator housing, the stator and stator housing undergo substantially different rates of radial thermal expansion when subject to temperature changes, with the stator housing typically changing more than the stator. The relative radial growth of the stator housing with respect to the stator causes mechanical stress on the housing, and may result in cracks in the housing.
Since the coefficients of axial thermal expansion of the stator and housing are typically different, the stator housing expands relative to the stator in an axial direction along the length of the stator and exerts an axial spreading force that undesirably decreases the axial load pressure holding the stator laminations together and undesirably stresses the adhesive bond of the stator laminations. If the axial thermal growth differential and resultant stresses are sufficient, the epoxy bond holding the stator laminations together may fracture resulting in delamination of the stator at elevated temperatures or the housing stress may exceed its allowable limit resulting in housing cracks at low temperatures.
Although existing axial retaining devices may be used to maintain the integrity of the stator laminations at all temperatures, such devices make the generator more complex, attribute to a reduction in performance, and cause additional mechanical stress on the stator housing in the axial direction, since the rate of axial growth of the stator housing is then constrained by the rate of axial growth of the stator.